1. Field of the Invention
The present invention relates to a manufacturing method of a display device, and more particularly to a manufacturing method of a display device in which a transparent substrate (protection cover) is bonded with a display panel via an adhesive.
2. Description of Related Art
A display device such as a liquid crystal display is used in various sizes ranging from small portable telephones to large TV.
FIG. 11 is a cross-sectional view for explaining the conventional liquid crystal display. In FIG. 11, a display panel PNL such as a liquid crystal display panel, for example, is incorporated into a casing CAS for the portable telephone.
In the liquid crystal display panel, the display panel PNL has a substrate SUB1, a substrate SUB2, a sealing material SL for bonding the substrate SUB1 and the substrate SUB2, a liquid crystal LC sealed into the inside surrounded by the substrate SUB1, the substrate SUB2 and the sealing material SL, a polarizer POLL disposed on the side of the substrate SUB1 opposite to the liquid crystal LC, and a polarizer POL2 disposed on the side of the substrate SUB2 opposite to the liquid crystal LC, for example.
The display panel PNL is disposed to overlap an opening portion provided in the casing CAS. In the opening portion of the casing CAS, a transparent substrate (protection cover) COV made of glass or acrylic (PMMA), for example, is bonded by a double-faced tape TAP or the like. And there was a space SP with an air layer interposed between the display panel PNL and the transparent substrate COV in the liquid crystal display as shown in FIG. 11.
Besides the liquid crystal display as shown in FIG. 11, there is the liquid crystal display in which the transparent substrate such as a reinforcing substrate is bonded with the liquid crystal display panel via an ultraviolet curable or heat curable adhesive, as described in patent document 1, for example. In this case, the patent document 1 discloses that to prevent bubbles from remaining, a high viscosity adhesive having a viscosity from 10,000 to 100,000 cP (1 cP=1 mPa·s) is formed on the periphery, and a low viscosity adhesive having a viscosity from 100 to 1,000 cP is coated inside it, whereby the bonding is made under vacuum (reduced pressure), so that bubbles can be extinguished by returning them to the atmospheric pressure even if small bubbles having a diameter of about 1 mm are entrained. And the viscosity at which bubbles are unlikely to be entrained is from 100 to 1,000 cP as described in the patent document 1.
Besides, regarding the technique for preventing bubbles from remaining, there are patent document 2 and patent document 3, for example. As described in the patent document 2, the heat curable adhesive and bubbles are arranged like a pattern, and heated to a first heating temperature at which the adhesive is not curable, and reduced in pressure (vacuum) to dissolve the bubbles, and thereafter cured at a second heating temperature by applying pressure. At this time, the patent document 2 discloses that the adhesive is made in dots or intersecting stripes (like a lattice), the viscosity of adhesive when coated is 1000 cp or more, and the viscosity of adhesive when heated at the first heating temperature is 100 cp or less.
Also, the patent document 3 discloses that when the bonding is made under vacuum so that bubbles may not remain, an ultraviolet curable adhesive resin for tentative fixing is disposed at the corner, and a heat curable adhesive resin for sealing is made like a line or point.
[Patent document 1] JP-A-10-254380
[Patent document 2] JP-A-2006-36865
[Patent document 3] JP-A-2005-243413